10.)a.)How many grams of solid ammonium chloride should be added to 0.500 L of a

Question

10.)a.)How many grams of solid ammonium chloride should be added to 0.500 L of a 0.237 M ammonia solution to prepare a buffer with a pH of 8.340 ?

grams ammonium chloride = ___________ g.

b.)How many grams of solid ammonium bromide should be added to 1.50 L of a 7.24×10-2M ammonia solution to prepare a buffer with a pH of 8.520 ?

grams ammonium bromide =____________ g.

c.)
How many grams of solid sodium cyanide should be added to 2.00 L of a 0.144 Mhydrocyanic acid solution to prepare a buffer with a pH of 10.038 ?

grams sodium cyanide = ____________ g.

Explanation / Answer

(a) pKb of ammonia = 4.74

pOH of the required solution = 14-8.34 = 5.66

pOH = pKb + log [salr/base]

5.66 = 4.74 + log [salt/0.237]

or, [salt] = 1.97 M

Moles of NH4Cl = 1.97 M * 0.5 L = 0.986 moles

mass of NH4Cl required = 0.986 * 53.49g/mol = 52.72 gm

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pOH = 5.48

5.48 = 4.74 + log [salt/7.24*10^-2]

or, [salt] = 0.397 M

moles of Nh4Br = 0.397 M * 1.5 L =0.597 moles

mass of NH4Br = 0.597 moles * 97.94 g/mol = 58.45 gm

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pka = 9.21

pH = pka + log [salt/acid]

10.038 = 9.21 + log [salt/0.144]

or, [salt] = 0.97 M

moles of NaCN = 0.97 M * 2L = 1.94 moles

mass of naCn = 1.94 moles * 49.01 g/mol =94.99 gm

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